Ming-Fu Lin (林明甫)

All Publications

• Non-linear enhancement of ultrafast X-ray diffraction through transient resonances. Nature communications Kuschel, S., Ho, P. J., Al Haddad, A., Zimmermann, F. F., Flueckiger, L., Ware, M. R., Duris, J., MacArthur, J. P., Lutman, A., Lin, M. F., Li, X., Nakahara, K., Aldrich, J. W., Walter, P., Young, L., Bostedt, C., Marinelli, A., Gorkhover, T. 2025; 16 (1): 847

  Abstract

  Diffraction-before-destruction imaging with ultrashort X-ray pulses can visualize non-equilibrium processes, such as chemical reactions, with sub-femtosecond precision in the native environment. Here, a nanospecimen diffracts a single X-ray flash before it disintegrates. The sample structure can be reconstructed from the coherent diffraction image (CDI). State-of-the-art X-ray snapshots lack high spatial resolution because of weak diffraction signal. Bleaching effects from photo-ionization significantly restrain image brightness scaling. We find that non-linear transient ion resonances can overcome this barrier if X-ray laser pulses are shorter than in most experiments. We compared snapshots from individual  ≈ 100 nm Xe nanoparticles as a function of pulse duration and incoming X-ray fluence. Our experimental results and Monte Carlo simulations suggest that transient resonances can increase ionic scattering cross sections significantly beyond literature values. This provides a novel avenue towards substantial improvement of the spatial resolution in CDI in combination with sub-femtosecond temporal precision at the nanoscale.

  View details for DOI 10.1038/s41467-025-56046-y

  View details for PubMedID 39833149

  View details for PubMedCentralID 3429598

• Probing Electronic Coherence between Core-Level Vacancies at Different Atomic Sites PHYSICAL REVIEW X Wang, J., Driver, T. 2025; 15: 011008

  View details for DOI 10.1103/PhysRevX.15.011008

• Ultrafast structural dynamics of UV photoexcited cis,cis-1,3-cyclooctadiene observed with time-resolved electron diffraction. Physical chemistry chemical physics : PCCP Muvva, S. B., Liu, Y., Chakraborty, P., Nunes, J. P., Attar, A. R., Bhattacharyya, S., Borne, K., Champenois, E. G., Goff, N., Hegazy, K., Hoffmann, M. C., Ji, F., Lin, M. F., Luo, D., Ma, L., Odate, A., Pathak, S., Rolles, D., Rudenko, A., Saha, S. K., Shen, X., Wang, X., Ware, M. R., Weathersby, S., Weber, P. M., Wilkin, K. J., Wolf, T. J., Xiong, Y., Xu, X., Yang, J., Matsika, S., Weinacht, T., Centurion, M. 2024

  Abstract

  Conjugated diene molecules are highly reactive upon photoexcitation and can relax through multiple reaction channels that depend on the position of the double bonds and the degree of molecular rigidity. Understanding the photoinduced dynamics of these molecules is crucial for establishing general rules governing the relaxation and product formation. Here, we investigate the femtosecond time-resolved photoinduced excited-state structural dynamics of cis,cis-1,3-cyclooctadiene, a large-flexible cyclic conjugated diene molecule, upon excitation with 200 nm using mega-electron-volt ultrafast electron diffraction and trajectory surface hopping dynamics simulations. We tracked the photoinduced structural changes from the Franck-Condon region through the conical intersection seam to the ground state. Our findings revealed a novel primary reaction coordinate involving ring distortion, where the ring stretches along one axis and compresses along the perpendicular axis. The nuclear wavepacket remains compact along this reaction coordinate until it reaches the conical intersection seam, and it rapidly spreads as it approaches the ground state, where multiple products are formed.

  View details for DOI 10.1039/d4cp02785j

  View details for PubMedID 39652083

• Design and performance of a magnetic bottle electron spectrometer for high-energy photoelectron spectroscopy. The Review of scientific instruments Borne, K., O'Neal, J. T., Wang, J., Isele, E., Obaid, R., Berrah, N., Cheng, X., Bucksbaum, P. H., James, J., Kamalov, A., Larsen, K. A., Li, X., Lin, M. F., Liu, Y., Marinelli, A., Summers, A. M., Thierstein, E., Wolf, T. J., Rolles, D., Walter, P., Cryan, J. P., Driver, T. 2024; 95 (12)

  Abstract

  We describe the design and performance of a magnetic bottle electron spectrometer (MBES) for high-energy electron spectroscopy. Our design features a 2 m long electron drift tube and electrostatic retardation lens, achieving sub-electronvolt (eV) electron kinetic energy resolution for high energy (several hundred eV) electrons with a close to 4π collection solid angle. A segmented anode electron detector enables the simultaneous collection of photoelectron spectra in high resolution and high collection efficiency modes. This versatile instrument is installed at the time-resolved molecular and optical sciences instrument at the Linac Coherent Light Source x-ray free-electron laser (XFEL). In this paper, we demonstrate its high resolution, collection efficiency, and spatial selectivity in measurements where it is coupled to an XFEL source. These combined characteristics are designed to enable high-resolution time-resolved measurements using x-ray photoelectron, absorption, and Auger-Meitner spectroscopy. We also describe the pervasive artifact in MBES time-of-flight spectra that arises from a periodic modulation in electron collection efficiency and present a robust analysis procedure for its removal.

  View details for DOI 10.1063/5.0223334

  View details for PubMedID 39704606

• Ultrafast Symmetry Control in Photoexcited Quantum Dots. Advanced materials (Deerfield Beach, Fla.) Guzelturk, B., Portner, J., Ondry, J., Ghanbarzadeh, S., Tarantola, M., Jeong, A., Field, T., Chandler, A. M., Wieman, E., Hopper, T. R., Watkins, N. E., Yu, J., Cheng, X., Lin, M. F., Luo, D., Kramer, P. L., Shen, X., Reid, A. H., Borkiewicz, O., Ruett, U., Zhang, X., Lindenberg, A. M., Ma, J., Schaller, R. D., Talapin, D. V., Cotts, B. L. 2024: e2414196

  Abstract

  Symmetry control is essential for realizing unconventional properties, such as ferroelectricity, nonlinear optical responses, and complex topological order, thus it holds promise for the design of emerging quantum and photonic systems. Nevertheless, fast and reversible control of symmetry in materials remains a challenge, especially for nanoscale systems. Here, reversible symmetry changes are unveiled in colloidal lead chalcogenide quantum dots on picosecond timescales. Using a combination of ultrafast electron diffraction and total X-ray scattering, in conjunction with atomic-scale structural modeling and first-principles calculations, it is revealed that symmetry-broken lead sulfide quantum dots restore to a centrosymmetric phase upon photoexcitation. The symmetry restoration is driven by photoexcited electronic carriers, which suppress lead off-centering for about 100 ps. Furthermore, the change in symmetry is closely correlated with the electronic properties, and the bandgap transiently red-shifts in the symmetry-restored quantum dots. Overall, this study elucidates reversible symmetry changes in colloidal quantum dots, and more broadly defines a new methodology to optically control symmetry in nanoscale systems on ultrafast timescales.

  View details for DOI 10.1002/adma.202414196

  View details for PubMedID 39584653

• Coupling to octahedral tilts in halide perovskite nanocrystals induces phonon-mediated attractive interactions between excitons. Nature physics Yazdani, N., Bodnarchuk, M. I., Bertolotti, F., Masciocchi, N., Fureraj, I., Guzelturk, B., Cotts, B. L., Zajac, M., Rainò, G., Jansen, M., Boehme, S. C., Yarema, M., Lin, M. F., Kozina, M., Reid, A., Shen, X., Weathersby, S., Wang, X., Vauthey, E., Guagliardi, A., Kovalenko, M. V., Wood, V., Lindenberg, A. M. 2024; 20 (1): 47-53

  Abstract

  Understanding the origin of electron-phonon coupling in lead halide perovskites is key to interpreting and leveraging their optical and electronic properties. Here we show that photoexcitation drives a reduction of the lead-halide-lead bond angles, a result of deformation potential coupling to low-energy optical phonons. We accomplish this by performing femtosecond-resolved, optical-pump-electron-diffraction-probe measurements to quantify the lattice reorganization occurring as a result of photoexcitation in nanocrystals of FAPbBr3. Our results indicate a stronger coupling in FAPbBr3 than CsPbBr3. We attribute the enhanced coupling in FAPbBr3 to its disordered crystal structure, which persists down to cryogenic temperatures. We find the reorganizations induced by each exciton in a multi-excitonic state constructively interfere, giving rise to a coupling strength that scales quadratically with the exciton number. This superlinear scaling induces phonon-mediated attractive interactions between excitations in lead halide perovskites.

  View details for DOI 10.1038/s41567-023-02253-7

  View details for PubMedID 38261834

  View details for PubMedCentralID PMC10791581

• Element-specific ultrafast lattice dynamics in FePt nanoparticles. Structural dynamics (Melville, N.Y.) Turenne, D., Vaskivskyi, I., Sokolowski-Tinten, K., Wang, X. J., Reid, A. H., Shen, X., Lin, M. F., Park, S., Weathersby, S., Kozina, M., Hoffmann, M. C., Wang, J., Sebesta, J., Takahashi, Y. K., Grånäs, O., Oppeneer, P. M., Dürr, H. A. 2024; 11 (6): 064501

  Abstract

  Light-matter interaction at the nanoscale in magnetic alloys and heterostructures is a topic of intense research in view of potential applications in high-density magnetic recording. While the element-specific dynamics of electron spins is directly accessible to resonant x-ray pulses with femtosecond time structure, the possible element-specific atomic motion remains largely unexplored. We use ultrafast electron diffraction (UED) to probe the temporal evolution of lattice Bragg peaks of FePt nanoparticles embedded in a carbon matrix following excitation by an optical femtosecond laser pulse. The diffraction interference between Fe and Pt sublattices enables us to demonstrate that the Fe mean square vibration amplitudes are significantly larger that those of Pt as expected from their different atomic mass. Both are found to increase as energy is transferred from the laser-excited electrons to the lattice. Contrary to this intuitive behavior, we observe a laser-induced lattice expansion that is larger for Pt than for Fe atoms during the first picosecond after laser excitation. This effect points to the strain-wave driven lattice expansion with the longitudinal acoustic Pt motion dominating that of Fe.

  View details for DOI 10.1063/4.0000260

  View details for PubMedID 39582608

  View details for PubMedCentralID PMC11585363

• Tracking dissociation pathways of nitrobenzene via mega-electron-volt ultrafast electron diffraction JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Hegazy, K., Bucksbaum, P., Centurion, M., Cryan, J., Li, R., Lin, M., Moore, B., Nunes, P., Shen, X., Weathersby, S., Yang, J., Wang, X., Wolf, T. 2024; 57 (19)

  View details for DOI 10.1088/1361-6455/ad7431

  View details for Web of Science ID 001306576300001

• UV-Induced Reaction Pathways in Bromoform Probed with Ultrafast Electron Diffraction. Journal of the American Chemical Society Hoffmann, L., Toulson, B. W., Yang, J., Saladrigas, C. A., Zong, A., Muvva, S. B., Figueira Nunes, J. P., Reid, A. H., Attar, A. R., Luo, D., Ji, F., Lin, M. F., Fan, Q., Weathersby, S. P., Shen, X., Wang, X., Wolf, T. J., Neumark, D. M., Leone, S. R., Zuerch, M. W., Centurion, M., Gessner, O. 2024

  Abstract

  For many chemical reactions, it remains notoriously difficult to predict and experimentally determine the rates and branching ratios between different reaction channels. This is particularly the case for reactions involving short-lived intermediates, whose observation requires ultrafast methods. The UV photochemistry of bromoform (CHBr3) is among the most intensely studied photoreactions. Yet, a detailed understanding of the chemical pathways leading to the production of atomic Br and molecular Br2 fragments has proven challenging. In particular, the role of isomerization and/or roaming and their competition with direct C-Br bond scission has been a matter of continued debate. Here, gas-phase ultrafast megaelectronvolt electron diffraction (MeV-UED) is used to directly study structural dynamics in bromoform after single 267 nm photon excitation with femtosecond temporal resolution. The results show unambiguously that isomerization contributes significantly to the early stages of the UV photochemistry of bromoform. In addition to direct C-Br bond breaking within <200 fs, formation of iso-CHBr3 (Br-CH-Br-Br) is observed on the same time scale and with an isomer lifetime of >1.1 ps. The branching ratio between direct dissociation and isomerization is determined to be 0.4 ± 0.2:0.6 ± 0.2, i.e., approximately 60% of molecules undergo isomerization within the first few hundred femtoseconds after UV excitation. The structure and time of formation of iso-CHBr3 compare favorably with the results of an ab initio molecular dynamics simulation. The lifetime and interatomic distances of the isomer are consistent with the involvement of a roaming reaction mechanism.

  View details for DOI 10.1021/jacs.4c07165

  View details for PubMedID 39374484

• Attosecond impulsive stimulated X-ray Raman scattering in liquid water. Science advances Alexander, O., Egun, F., Rego, L., Gutierrez, A. M., Garratt, D., Cardenes, G. A., Nogueira, J. J., Lee, J. P., Zhao, K., Wang, R. P., Ayuso, D., Barnard, J. C., Beauvarlet, S., Bucksbaum, P. H., Cesar, D., Coffee, R., Duris, J., Frasinski, L. J., Huse, N., Kowalczyk, K. M., Larsen, K. A., Matthews, M., Mukamel, S., O'Neal, J. T., Penfold, T., Thierstein, E., Tisch, J. W., Turner, J. R., Vogwell, J., Driver, T., Berrah, N., Lin, M. F., Dakovski, G. L., Moeller, S. P., Cryan, J. P., Marinelli, A., Picón, A., Marangos, J. P. 2024; 10 (39): eadp0841

  Abstract

  We report the measurement of impulsive stimulated x-ray Raman scattering in neutral liquid water. An attosecond pulse drives the excitations of an electronic wavepacket in water molecules. The process comprises two steps: a transition to core-excited states near the oxygen atoms accompanied by transition to valence-excited states. Thus, the wavepacket is impulsively created at a specific atomic site within a few hundred attoseconds through a nonlinear interaction between the water and the x-ray pulse. We observe this nonlinear signature in an intensity-dependent Stokes Raman sideband at 526 eV. Our measurements are supported by our state-of-the-art calculations based on the polarization response of water dimers in bulk solvation and propagation of attosecond x-ray pulses at liquid density.

  View details for DOI 10.1126/sciadv.adp0841

  View details for PubMedID 39321305

• Attosecond delays in X-ray molecular ionization. Nature Driver, T., Mountney, M., Wang, J., Ortmann, L., Al-Haddad, A., Berrah, N., Bostedt, C., Champenois, E. G., DiMauro, L. F., Duris, J., Garratt, D., Glownia, J. M., Guo, Z., Haxton, D., Isele, E., Ivanov, I., Ji, J., Kamalov, A., Li, S., Lin, M. F., Marangos, J. P., Obaid, R., O'Neal, J. T., Rosenberger, P., Shivaram, N. H., Wang, A. L., Walter, P., Wolf, T. J., Wörner, H. J., Zhang, Z., Bucksbaum, P. H., Kling, M. F., Landsman, A. S., Lucchese, R. R., Emmanouilidou, A., Marinelli, A., Cryan, J. P. 2024; 632 (8026): 762-767

  Abstract

  The photoelectric effect is not truly instantaneous but exhibits attosecond delays that can reveal complex molecular dynamics1-7. Sub-femtosecond-duration light pulses provide the requisite tools to resolve the dynamics of photoionization8-12. Accordingly, the past decade has produced a large volume of work on photoionization delays following single-photon absorption of an extreme ultraviolet photon. However, the measurement of time-resolved core-level photoionization remained out of reach. The required X-ray photon energies needed for core-level photoionization were not available with attosecond tabletop sources. Here we report measurements of the X-ray photoemission delay of core-level electrons, with unexpectedly large delays, ranging up to 700 as in NO near the oxygen K-shell threshold. These measurements exploit attosecond soft X-ray pulses from a free-electron laser to scan across the entire region near the K-shell threshold. Furthermore, we find that the delay spectrum is richly modulated, suggesting several contributions, including transient trapping of the photoelectron owing to shape resonances, collisions with the Auger-Meitner electron that is emitted in the rapid non-radiative relaxation of the molecule and multi-electron scattering effects. The results demonstrate how X-ray attosecond experiments, supported by comprehensive theoretical modelling, can unravel the complex correlated dynamics of core-level photoionization.

  View details for DOI 10.1038/s41586-024-07771-9

  View details for PubMedID 39169246

  View details for PubMedCentralID 7399650

• Orbital-selective time-domain signature of nematicity dynamics in the charge-density-wave phase of La1.65Eu0.2Sr0.15CuO4. Proceedings of the National Academy of Sciences of the United States of America Bluschke, M., Gupta, N. K., Jang, H., Husain, A. A., Lee, B., Kim, M., Na, M., Dos Remedios, B., Smit, S., Moen, P., Park, S. Y., Kim, M., Jang, D., Choi, H., Sutarto, R., Reid, A. H., Dakovski, G. L., Coslovich, G., Nguyen, Q. L., Burdet, N. G., Lin, M. F., Revcolevschi, A., Park, J. H., Geck, J., Turner, J. J., Damascelli, A., Hawthorn, D. G. 2024; 121 (23): e2400727121

  Abstract

  Understanding the interplay between charge, nematic, and structural ordering tendencies in cuprate superconductors is critical to unraveling their complex phase diagram. Using pump-probe time-resolved resonant X-ray scattering on the (0 0 1) Bragg peak at the Cu [Formula: see text] and O [Formula: see text] resonances, we investigate nonequilibrium dynamics of [Formula: see text] nematic order and its association with both charge density wave (CDW) order and lattice dynamics in La[Formula: see text]Eu[Formula: see text]Sr[Formula: see text]CuO[Formula: see text]. The orbital selectivity of the resonant X-ray scattering cross-section allows nematicity dynamics associated with the planar O 2[Formula: see text] and Cu 3[Formula: see text] states to be distinguished from the response of anisotropic lattice distortions. A direct time-domain comparison of CDW translational-symmetry breaking and nematic rotational-symmetry breaking reveals that these broken symmetries remain closely linked in the photoexcited state, consistent with the stability of CDW topological defects in the investigated pump fluence regime.

  View details for DOI 10.1073/pnas.2400727121

  View details for PubMedID 38819998

• Photo-induced structural dynamics of o-nitrophenol by ultrafast electron diffraction. Physical chemistry chemical physics : PCCP Nunes, J. P., Williams, M., Yang, J., Wolf, T. J., Rankine, C. D., Parrish, R., Moore, B., Wilkin, K., Shen, X., Lin, M. F., Hegazy, K., Li, R., Weathersby, S., Martinez, T. J., Wang, X. J., Centurion, M. 2024

  Abstract

  The photo-induced dynamics of o-nitrophenol, particularly its photolysis, has garnered significant scientific interest as a potential source of nitrous acid in the atmosphere. Although the photolysis products and preceding photo-induced electronic structure dynamics have been investigated extensively, the nuclear dynamics accompanying the non-radiative relaxation of o-nitrophenol on the ultrafast timescale, which include an intramolecular proton transfer step, have not been experimentally resolved. Herein, we present a direct observation of the ultrafast nuclear motions mediating photo-relaxation using ultrafast electron diffraction. This work spatiotemporally resolves the loss of planarity which enables access to a conical intersection between the first excited state and the ground state after the proton transfer step, on the femtosecond timescale and with sub-Angstrom resolution. Our observations, supported by ab initio multiple spawning simulations, provide new insights into the proton transfer mediated relaxation mechanism in o-nitrophenol.

  View details for DOI 10.1039/d3cp06253h

  View details for PubMedID 38764355

• Terawatt-scale attosecond X-ray pulses from a cascaded superradiant free-electron laser NATURE PHOTONICS Franz, P., Li, S., Driver, T., Robles, R. R., Cesar, D., Isele, E., Guo, Z., Wang, J., Duris, J. P., Larsen, K., Glownia, J. M., Cheng, X., Hoffmann, M. C., Li, X., Lin, M., Kamalov, A., Obaid, R., Summers, A., Sudar, N., Thierstein, E., Zhang, Z., Kling, M. F., Huang, Z., Cryan, J. P., Marinelli, A. 2024

  View details for DOI 10.1038/s41566-024-01427-w

  View details for Web of Science ID 001220935700001

• Experimental demonstration of attosecond pump-probe spectroscopy with an X-ray free-electron laser NATURE PHOTONICS Guo, Z., Driver, T., Beauvarlet, S., Cesar, D., Duris, J., Franz, P. L., Alexander, O., Bohler, D., Bostedt, C., Averbukh, V., Cheng, X., Dimauro, L. F., Doumy, G., Forbes, R., Gessner, O., Glownia, J. M., Isele, E., Kamalov, A., Larsen, K. A., Li, S., Li, X., Lin, M., Mccracken, G. A., Obaid, R., O'Neal, J. T., Robles, R. R., Rolles, D., Ruberti, M., Rudenko, A., Slaughter, D. S., Sudar, N. S., Thierstein, E., Tuthill, D., Ueda, K., Wang, E., Wang, A. L., Wang, J., Weber, T., Wolf, T. A., Young, L., Zhang, Z., Bucksbaum, P. H., Marangos, J. P., Kling, M. F., Huang, Z., Walter, P., Inhester, L., Berrah, N., Cryan, J. P., Marinelli, A. 2024

  View details for DOI 10.1038/s41566-024-01419-w

  View details for Web of Science ID 001200371400001

• Attosecond-pump attosecond-probe x-ray spectroscopy of liquid water. Science (New York, N.Y.) Li, S., Lu, L., Bhattacharyya, S., Pearce, C., Li, K., Nienhuis, E. T., Doumy, G., Schaller, R. D., Moeller, S., Lin, M. F., Dakovski, G., Hoffman, D. J., Garratt, D., Larsen, K. A., Koralek, J. D., Hampton, C. Y., Cesar, D., Duris, J., Zhang, Z., Sudar, N., Cryan, J. P., Marinelli, A., Li, X., Inhester, L., Santra, R., Young, L. 2024: eadn6059

  Abstract

  Attosecond-pump/attosecond-probe experiments have long been sought as the most straightforward method to observe electron dynamics in real time. Although numerous successes have been achieved with overlapped near infrared femtosecond and extreme ultraviolet attosecond pulses combined with theory, true attosecond-pump/attosecond-probe experiments have been limited. We used a synchronized attosecond x-ray pulse pair from an x-ray free electron laser to study the electronic response to valence ionization in liquid water via all x-ray attosecond transient absorption spectroscopy (AX-ATAS). Our analysis showed that the AX-ATAS response is confined to the subfemtosecond timescale, eliminating any hydrogen atom motion and demonstrating experimentally that the 1b1 splitting in the x-ray emission spectrum is related to dynamics and is not evidence for two structural motifs in ambient liquid water.

  View details for DOI 10.1126/science.adn6059

  View details for PubMedID 38359104

• Monitoring the Evolution of Relative Product Populations at Early Times during a Photochemical Reaction. Journal of the American Chemical Society Figueira Nunes, J. P., Ibele, L. M., Pathak, S., Attar, A. R., Bhattacharyya, S., Boll, R., Borne, K., Centurion, M., Erk, B., Lin, M., Forbes, R. J., Goff, N., Hansen, C. S., Hoffmann, M., Holland, D. M., Ingle, R. A., Luo, D., Muvva, S. B., Reid, A. H., Rouzee, A., Rudenko, A., Saha, S. K., Shen, X., Venkatachalam, A. S., Wang, X., Ware, M. R., Weathersby, S. P., Wilkin, K., Wolf, T. J., Xiong, Y., Yang, J., Ashfold, M. N., Rolles, D., Curchod, B. F. 2024

  Abstract

  Identifying multiple rival reaction products and transient species formed during ultrafast photochemical reactions and determining their time-evolving relative populations are key steps toward understanding and predicting photochemical outcomes. Yet, most contemporary ultrafast studies struggle with clearly identifying and quantifying competing molecular structures/species among the emerging reaction products. Here, we show that mega-electronvolt ultrafast electron diffraction in combination with ab initio molecular dynamics calculations offer a powerful route to determining time-resolved populations of the various isomeric products formed after UV (266 nm) excitation of the five-membered heterocyclic molecule 2(5H)-thiophenone. This strategy provides experimental validation of the predicted high (50%) yield of an episulfide isomer containing a strained three-membered ring within 1 ps of photoexcitation and highlights the rapidity of interconversion between the rival highly vibrationally excited photoproducts in their ground electronic state.

  View details for DOI 10.1021/jacs.3c13046

  View details for PubMedID 38317439

• Prediction on X-ray output of free electron laser based on artificial neural networks NATURE COMMUNICATIONS Li, K., Zhou, G., Liu, Y., Wu, J., Lin, M., Cheng, X., Lutman, A. A., Seaberg, M., Smith, H., Kakhandiki, P. A., Sakdinawat, A. 2023; 14 (1): 7183

  Abstract

  Knowledge of x-ray free electron lasers' (XFELs) pulse characteristics delivered to a sample is crucial for ensuring high-quality x-rays for scientific experiments. XFELs' self-amplified spontaneous emission process causes spatial and spectral variations in x-ray pulses entering a sample, which leads to measurement uncertainties for experiments relying on multiple XFEL pulses. Accurate in-situ measurements of x-ray wavefront and energy spectrum incident upon a sample poses challenges. Here we address this by developing a virtual diagnostics framework using an artificial neural network (ANN) to predict x-ray photon beam properties from electron beam properties. We recorded XFEL electron parameters while adjusting the accelerator's configurations and measured the resulting x-ray wavefront and energy spectrum shot-to-shot. Training the ANN with this data enables effective prediction of single-shot or average x-ray beam output based on XFEL undulator and electron parameters. This demonstrates the potential of utilizing ANNs for virtual diagnostics linking XFEL electron and photon beam properties.

  View details for DOI 10.1038/s41467-023-42573-z

  View details for Web of Science ID 001096629100001

  View details for PubMedID 37935675

  View details for PubMedCentralID PMC10630459

• Compact single-shot soft X-ray photon spectrometer for free-electron laser diagnostics OPTICS EXPRESS Larsen, K. A., Borne, K., Obaid, R., Kamalov, A., Liu, Y., Cheng, X., James, J., Driver, T., Li, K., Liu, Y., Sakdinawat, A., David, C., Wolf, T. A., Cryan, J. P., Walter, P., Lin, M. 2023; 31 (22): 35822-35834

  Abstract

  The photon spectrum from free-electron laser (FEL) light sources offers valuable information in time-resolved experiments and machine optimization in the spectral and temporal domains. We have developed a compact single-shot photon spectrometer to diagnose soft X-ray spectra. The spectrometer consists of an array of off-axis Fresnel zone plates (FZP) that act as transmission-imaging gratings, a Ce:YAG scintillator, and a microscope objective to image the scintillation target onto a two-dimensional imaging detector. This spectrometer operates in segmented energy ranges which covers tens of electronvolts for each absorption edge associated with several atomic constituents: carbon, nitrogen, oxygen, and neon. The spectrometer's performance is demonstrated at a repetition rate of 120 Hz, but our detection scheme can be easily extended to 200 kHz spectral collection by employing a fast complementary metal oxide semiconductor (CMOS) line-scan camera to detect the light from the scintillator. This compact photon spectrometer provides an opportunity for monitoring the spectrum downstream of an endstation in a limited space environment with sub-electronvolt energy resolution.

  View details for DOI 10.1364/OE.502105

  View details for Web of Science ID 001106418000001

  View details for PubMedID 38017746

• Femtosecond Electronic and Hydrogen Structural Dynamics in Ammonia Imaged with Ultrafast Electron Diffraction. Physical review letters Champenois, E. G., List, N. H., Ware, M., Britton, M., Bucksbaum, P. H., Cheng, X., Centurion, M., Cryan, J. P., Forbes, R., Gabalski, I., Hegazy, K., Hoffmann, M. C., Howard, A. J., Ji, F., Lin, M. F., Nunes, J. P., Shen, X., Yang, J., Wang, X., Martinez, T. J., Wolf, T. J. 2023; 131 (14): 143001

  Abstract

  Directly imaging structural dynamics involving hydrogen atoms by ultrafast diffraction methods is complicated by their low scattering cross sections. Here we demonstrate that megaelectronvolt ultrafast electron diffraction is sufficiently sensitive to follow hydrogen dynamics in isolated molecules. In a study of the photodissociation of gas phase ammonia, we simultaneously observe signatures of the nuclear and corresponding electronic structure changes resulting from the dissociation dynamics in the time-dependent diffraction. Both assignments are confirmed by ab initio simulations of the photochemical dynamics and the resulting diffraction observable. While the temporal resolution of the experiment is insufficient to resolve the dissociation in time, our results represent an important step towards the observation of proton dynamics in real space and time.

  View details for DOI 10.1103/PhysRevLett.131.143001

  View details for PubMedID 37862660

• Rehybridization dynamics into the pericyclic minimum of an electrocyclic reaction imaged in real-time. Nature communications Liu, Y., Sanchez, D. M., Ware, M. R., Champenois, E. G., Yang, J., Nunes, J. P., Attar, A., Centurion, M., Cryan, J. P., Forbes, R., Hegazy, K., Hoffmann, M. C., Ji, F., Lin, M., Luo, D., Saha, S. K., Shen, X., Wang, X. J., Martinez, T. J., Wolf, T. J. 2023; 14 (1): 2795

  Abstract

  Electrocyclic reactions are characterized by the concerted formation and cleavage of both sigma and pi bonds through a cyclic structure. This structure is known as a pericyclic transition state for thermal reactions and a pericyclic minimum in the excited state for photochemical reactions. However, the structure of the pericyclic geometry has yet to be observed experimentally. We use a combination of ultrafast electron diffraction and excited state wavepacket simulations to image structural dynamics through the pericyclic minimum of a photochemical electrocyclic ring-opening reaction in the molecule alpha-terpinene. The structural motion into the pericyclic minimum is dominated by rehybridization of two carbon atoms, which is required for the transformation from two to three conjugated pi bonds. The sigma bond dissociation largely happens after internal conversion from the pericyclic minimum to the electronic ground state. These findings may be transferrable to electrocyclic reactions in general.

  View details for DOI 10.1038/s41467-023-38513-6

  View details for PubMedID 37202402

• Understanding and Controlling Photothermal Responses in MXenes. Nano letters Guzelturk, B., Kamysbayev, V., Wang, D., Hu, H., Li, R., King, S. B., Reid, A. H., Lin, M., Wang, X., Walko, D. A., Zhang, X., Lindenberg, A., Talapin, D. V. 2023

  Abstract

  MXenes have the potential for efficient light-to-heat conversion in photothermal applications. To effectively utilize MXenes in such applications, it is important to understand the underlying nonequilibrium processes, including electron-phonon and phonon-phonon couplings. Here, we use transient electron and X-ray diffraction to investigate the heating and cooling of photoexcited MXenes at femtosecond to nanosecond time scales. Our results show extremely strong electron-phonon coupling in Ti3C2-based MXenes, resulting in lattice heating within a few hundred femtoseconds. We also systematically study heat dissipation in MXenes with varying film thicknesses, chemical surface terminations, flake sizes, and annealing conditions. We find that the thermal boundary conductance (TBC) governs the thermal relaxation in films thinner than the optical penetration depth. We achieve a 2-fold enhancement of the TBC, reaching 20 MW m-2 K-1, by controlling the flake size or chemical surface termination, which is promising for engineering heat dissipation in photothermal and thermoelectric applications of the MXenes.

  View details for DOI 10.1021/acs.nanolett.2c05001

  View details for PubMedID 36917456

• Ultrafast relaxation of lattice distortion in two-dimensional perovskites NATURE PHYSICS Zhang, H., Li, W., Essman, J., Quarti, C., Metcalf, I., Chiang, W., Sidhik, S., Hou, J., Fehr, A., Attar, A., Lin, M., Britz, A., Shen, X., Link, S., Wang, X., Bergmann, U., Kanatzidis, M. G., Katan, C., Even, J., Blancon, J., Mohite, A. D. 2023

  View details for DOI 10.1038/s41567-022-01903-6

  View details for Web of Science ID 000922938200003

• CH12: Imaging Ultrafast Structural Dynamics with Megaelectronvolt Ultrafast Electron Diffraction Structural Dynamics with X-ray and Electron Scattering Lin, M., Wolf, T., Shen, X., Reid, A. edited by Armini, K., Rouzée, A., Vrakking, M. J. Royal Society of Chemistry. 2023

  View details for DOI 10.1039/BK9781837671564-00466

• Large Exchange Coupling Between Localized Spins and Topological Bands in Magnetic Topological Insulator MnBi2 Te4. Advanced materials (Deerfield Beach, Fla.) Padmanabhan, H., Stoica, V. A., Kim, P. K., Poore, M., Yang, T., Shen, X., Reid, A. H., Lin, M., Park, S., Yang, J., Hugo Wang, H., Koocher, N. Z., Puggioni, D., Georgescu, A. B., Min, L., Lee, S. H., Mao, Z., Rondinelli, J. M., Lindenberg, A. M., Chen, L., Wang, X., Averitt, R. D., Freeland, J. W., Gopalan, V. 2022: e2202841

  Abstract

  Magnetism in topological materials creates phases exhibiting quantized transport phenomena with potential technological applications. The emergence of such phases relies on strong interaction between localized spins and the topological bands, and the consequent formation of an exchange gap. However, this remains experimentally unquantified in intrinsic magnetic topological materials. Here, this interaction is quantified in MnBi2 Te4 , an intrinsic antiferromagnetic topological insulator. To achieve this, a multimodal ultrafast approach is employed to interrogate optically induced nonequilibrium spin dynamics. Momentum-resolved ultrafast electron scattering and magneto-optic measurements show that Bi-Te p-like states comprising the bulk topological bands demagnetize via electron-phonon scattering at picosecond timescales. Localized Mn 3d spins, probed by ultrafast resonant X-ray scattering, are found to disorder concurrently with the p-like spins, despite being energetically decoupled from the optical excitation. These results, together with atomistic simulations, reveal that the exchange coupling between localized spins and the bulk topological bands is at least 100 times larger than the primary superexchange interaction, implying an optimal exchange gap of at least 25 meV in the topological surface states. By directly quantifying this exchange coupling, the study validates the materials-by-design strategy of utilizing localized magnetic order to create and manipulate magnetic topological phases, spanning static to ultrafast timescales. This article is protected by copyright. All rights reserved.

  View details for DOI 10.1002/adma.202202841

  View details for PubMedID 36189841

• The DREAM Endstation at the Linac Coherent Light Source APPLIED SCIENCES-BASEL Walter, P., Holmes, M., Obaid, R., Amores, L., Cheng, X., Cryan, J. P., Glownia, J. M., Li, X., Lin, M., Ng, M., Robinson, J., Shivaram, N., Yin, J., Fritz, D., James, J., Castagna, J., Osipov, T. 2022; 12 (20)

  View details for DOI 10.3390/app122010534

  View details for Web of Science ID 000874182200001

• The time-resolved atomic, molecular and optical science instrument at the Linac Coherent Light Source. Journal of synchrotron radiation Walter, P., Osipov, T., Lin, M. F., Cryan, J., Driver, T., Kamalov, A., Marinelli, A., Robinson, J., Seaberg, M. H., Wolf, T. J., Aldrich, J., Brown, N., Champenois, E. G., Cheng, X., Cocco, D., Conder, A., Curiel, I., Egger, A., Glownia, J. M., Heimann, P., Holmes, M., Johnson, T., Lee, L., Li, X., Moeller, S., Morton, D. S., Ng, M. L., Ninh, K., O'Neal, J. T., Obaid, R., Pai, A., Schlotter, W., Shepard, J., Shivaram, N., Stefan, P., Van, X., Wang, A. L., Wang, H., Yin, J., Yunus, S., Fritz, D., James, J., Castagna, J. C. 2022; 29 (Pt 4): 957-968

  Abstract

  The newly constructed time-resolved atomic, molecular and optical science instrument (TMO) is configured to take full advantage of both linear accelerators at SLAC National Accelerator Laboratory, the copper accelerator operating at a repetition rate of 120 Hz providing high per-pulse energy as well as the superconducting accelerator operating at a repetition rate of about 1 MHz providing high average intensity. Both accelerators power a soft X-ray free-electron laser with the new variable-gap undulator section. With this flexible light source, TMO supports many experimental techniques not previously available at LCLS and will have two X-ray beam focus spots in line. Thereby, TMO supports atomic, molecular and optical, strong-field and nonlinear science and will also host a designated new dynamic reaction microscope with a sub-micrometer X-ray focus spot. The flexible instrument design is optimized for studying ultrafast electronic and molecular phenomena and can take full advantage of the sub-femtosecond soft X-ray pulse generation program.

  View details for DOI 10.1107/S1600577522004283

  View details for PubMedID 35787561

• Multichannel photodissociation dynamics in CS2 studied by ultrafast electron diffraction. Physical chemistry chemical physics : PCCP Razmus, W. O., Acheson, K., Bucksbaum, P., Centurion, M., Champenois, E., Gabalski, I., Hoffman, M. C., Howard, A., Lin, M., Liu, Y., Nunes, P., Saha, S., Shen, X., Ware, M., Warne, E. M., Weinacht, T., Wilkin, K., Yang, J., Wolf, T. J., Kirrander, A., Minns, R. S., Forbes, R. 2022

  Abstract

  The structural dynamics of photoexcited gas-phase carbon disulfide (CS2) molecules are investigated using ultrafast electron diffraction. The dynamics were triggered by excitation of the optically bright 1B2(1Sigmau+) state by an ultraviolet femtosecond laser pulse centred at 200 nm. In accordance with previous studies, rapid vibrational motion facilitates a combination of internal conversion and intersystem crossing to lower-lying electronic states. Photodissociation via these electronic manifolds results in the production of CS fragments in the electronic ground state and dissociated singlet and triplet sulphur atoms. The structural dynamics are extracted from the experiment using a trajectory-fitting filtering approach, revealing the main characteristics of the singlet and triplet dissociation pathways. Finally, the effect of the time-resolution on the experimental signal is considered and an outlook to future experiments provided.

  View details for DOI 10.1039/d2cp01268e

  View details for PubMedID 35707953

• Enhanced charge density wave coherence in a light-quenched, high-temperature superconductor. Science (New York, N.Y.) Wandel, S., Boschini, F., da Silva Neto, E. H., Shen, L., Na, M. X., Zohar, S., Wang, Y., Welch, S. B., Seaberg, M. H., Koralek, J. D., Dakovski, G. L., Hettel, W., Lin, M., Moeller, S. P., Schlotter, W. F., Reid, A. H., Minitti, M. P., Boyle, T., He, F., Sutarto, R., Liang, R., Bonn, D., Hardy, W., Kaindl, R. A., Hawthorn, D. G., Lee, J., Kemper, A. F., Damascelli, A., Giannetti, C., Turner, J. J., Coslovich, G. 2022; 376 (6595): 860-864

  Abstract

  Superconductivity and charge density waves (CDWs) are competitive, yet coexisting, orders in cuprate superconductors. To understand their microscopic interdependence, a probe capable of discerning their interaction on its natural length and time scale is necessary. We use ultrafast resonant soft x-ray scattering to track the transient evolution of CDW correlations in YBa2Cu3O6+x after the quench of superconductivity by an infrared laser pulse. We observe a nonthermal response of the CDW order characterized by a near doubling of the correlation length within 1 picosecond of the superconducting quench. Our results are consistent with a model in which the interaction between superconductivity and CDWs manifests inhomogeneously through disruption of spatial coherence, with superconductivity playing the dominant role in stabilizing CDW topological defects, such as discommensurations.

  View details for DOI 10.1126/science.abd7213

  View details for PubMedID 35587968

• Attosecond coherent electron motion in Auger-Meitner decay. Science (New York, N.Y.) Li, S., Driver, T., Rosenberger, P., Champenois, E. G., Duris, J., Al-Haddad, A., Averbukh, V., Barnard, J. C., Berrah, N., Bostedt, C., Bucksbaum, P. H., Coffee, R. N., DiMauro, L. F., Fang, L., Garratt, D., Gatton, A., Guo, Z., Hartmann, G., Haxton, D., Helml, W., Huang, Z., LaForge, A. C., Kamalov, A., Knurr, J., Lin, M., Lutman, A. A., MacArthur, J. P., Marangos, J. P., Nantel, M., Natan, A., Obaid, R., O'Neal, J. T., Shivaram, N. H., Schori, A., Walter, P., Wang, A. L., Wolf, T. J., Zhang, Z., Kling, M. F., Marinelli, A., Cryan, J. P. 1800: eabj2096

  Abstract

  [Figure: see text].

  View details for DOI 10.1126/science.abj2096

  View details for PubMedID 34990213

• The X-ray Focusing System at the Time-Resolved AMO Instrument Synchrotron Radiation News Seaberg, M., et al 2022; 35 (2): 20-28

  View details for DOI 10.1080/08940886.2022.2066416

• Aggregation of solutes in bosonic versus fermionic quantum fluids. Science advances Feinberg, A. J., Verma, D., O'Connell-Lopez, S. M., Erukala, S., Tanyag, R. M., Pang, W., Saladrigas, C. A., Toulson, B. W., Borgwardt, M., Shivaram, N., Lin, M. F., Al Haddad, A., Jäger, W., Bostedt, C., Walter, P., Gessner, O., Vilesov, A. F. 2021; 7 (50): eabk2247

  Abstract

  [Figure: see text].

  View details for DOI 10.1126/sciadv.abk2247

  View details for PubMedID 34890219

• Carrier-Specific Hot Phonon Bottleneck in CH3NH3PbI3 Revealed by Femtosecond XUV Absorption. Journal of the American Chemical Society Verkamp, M., Leveillee, J., Sharma, A., Lin, M. F., Schleife, A., Vura-Weis, J. 2021; 143 (48): 20176-20182

  Abstract

  Femtosecond carrier cooling in the organohalide perovskite semiconductor CH3NH3PbI3 is measured using extreme ultraviolet (XUV) and optical transient absorption spectroscopy. XUV absorption between 44 and 58 eV measures transitions from the I 4d core to the valence and conduction bands and gives distinct signals for hole and electron dynamics. The core-to-valence-band signal directly maps the photoexcited hole distribution and provides a quantitative measurement of the hole temperature. The combination of XUV and optical probes reveals that upon excitation at 400 nm, the initial hole distribution is 3.5 times hotter than the electron distribution. At an initial carrier density of 1.4 × 1020 cm-3 both carriers are subject to a hot phonon bottleneck, but at 4.2 × 1019 cm-3 the holes cool to less than 1000 K within 400 fs. This result places significant constraints on the use of organohalide perovskites in hot-carrier photovoltaics.

  View details for DOI 10.1021/jacs.1c07817

  View details for PubMedID 34813692

• Imaging the short-lived hydroxyl-hydronium pair in ionized liquid water. Science (New York, N.Y.) Lin, M., Singh, N., Liang, S., Mo, M., Nunes, J. P., Ledbetter, K., Yang, J., Kozina, M., Weathersby, S., Shen, X., Cordones, A. A., Wolf, T. J., Pemmaraju, C. D., Ihme, M., Wang, X. J. 2021; 374 (6563): 92-95

  Abstract

  [Figure: see text].

  View details for DOI 10.1126/science.abg3091

  View details for PubMedID 34591617

• Spontaneous fluctuations in a magnetic Fe/Gd skyrmion lattice PHYSICAL REVIEW RESEARCH Seaberg, M. H., Holladay, B., Montoya, S. A., Zheng, X. Y., Lee, J. T., Reid, A. H., Koralek, J. D., Shen, L., Esposito, Coslovich, G., Walter, P., Zohar, S., Thampy, Lin, M. F., Hart, P., Nakahara, K., Streubel, R., Kevan, S. D., Fischer, P., Colocho, W., Lutman, A., Decker, F., Fullerton, E. E., Dunne, M., Roy, S., Sinha, S. K., Turner, J. J. 2021; 3 (3)

  View details for DOI 10.1103/PhysRevResearch.3.033249

  View details for Web of Science ID 000705659700001

• Direct observation of ultrafast hydrogen bond strengthening in liquid water. Nature Yang, J., Dettori, R., Nunes, J. P., List, N. H., Biasin, E., Centurion, M., Chen, Z., Cordones, A. A., Deponte, D. P., Heinz, T. F., Kozina, M. E., Ledbetter, K., Lin, M., Lindenberg, A. M., Mo, M., Nilsson, A., Shen, X., Wolf, T. J., Donadio, D., Gaffney, K. J., Martinez, T. J., Wang, X. 2021; 596 (7873): 531-535

  Abstract

  Water is one of the most important, yet least understood, liquids in nature. Many anomalous properties of liquid water originate from its well-connected hydrogen bond network1, including unusually efficient vibrational energy redistribution and relaxation2. An accurate description of the ultrafast vibrational motion of water molecules is essential for understanding the nature of hydrogen bonds and many solution-phase chemical reactions. Most existing knowledge of vibrational relaxation in water is built upon ultrafast spectroscopy experiments2-7. However, these experiments cannot directly resolve the motion of the atomic positions and require difficult translation of spectral dynamics into hydrogen bond dynamics. Here, we measure the ultrafast structural response to the excitation of the OH stretching vibration in liquid water with femtosecond temporal and atomic spatial resolution using liquid ultrafast electron scattering. We observed a transient hydrogen bond contraction of roughly 0.04A on a timescale of 80 femtoseconds, followed by a thermalization on a timescale of approximately 1 picosecond. Molecular dynamics simulations reveal the need to treat the distribution of the shared proton in the hydrogen bond quantum mechanically to capture the structural dynamics on femtosecond timescales. Our experiment and simulations unveil the intermolecular character of the water vibration preceding the relaxation of the OH stretch.

  View details for DOI 10.1038/s41586-021-03793-9

  View details for PubMedID 34433948

• Probing the interplay between lattice dynamics and short-range magnetic correlations in CuGeO3 with femtosecond RIXS NPJ QUANTUM MATERIALS Paris, E., Nicholson, C. W., Johnston, S., Tseng, Y., Rumo, M., Coslovich, G., Zohar, S., Lin, M. F., Strocov, V. N., Saint-Martin, R., Revcolevschi, A., Kemper, A., Schlotter, W., Dakovski, G. L., Monney, C., Schmitt, T. 2021; 6 (1)

  View details for DOI 10.1038/s41535-021-00350-5

  View details for Web of Science ID 000653043300001

• Dynamic lattice distortions driven by surface trapping in semiconductor nanocrystals. Nature communications Guzelturk, B., Cotts, B. L., Jasrasaria, D., Philbin, J. P., Hanifi, D. A., Koscher, B. A., Balan, A. D., Curling, E., Zajac, M., Park, S., Yazdani, N., Nyby, C., Kamysbayev, V., Fischer, S., Nett, Z., Shen, X., Kozina, M. E., Lin, M., Reid, A. H., Weathersby, S. P., Schaller, R. D., Wood, V., Wang, X., Dionne, J. A., Talapin, D. V., Alivisatos, A. P., Salleo, A., Rabani, E., Lindenberg, A. M. 2021; 12 (1): 1860

  Abstract

  Nonradiative processes limit optoelectronic functionality of nanocrystals and curb their device performance. Nevertheless, the dynamic structural origins of nonradiative relaxations in such materials are not understood. Here, femtosecond electron diffraction measurements corroborated by atomistic simulations uncover transient lattice deformations accompanying radiationless electronic processes in colloidal semiconductor nanocrystals. Investigation of the excitation energy dependence in a core/shell system shows that hot carriers created by a photon energy considerably larger than the bandgap induce structural distortions at nanocrystal surfaces on few picosecond timescales associated with the localization of trapped holes. On the other hand, carriers created by a photon energy close to the bandgap of the core in the same system result in transient lattice heating that occurs on a much longer 200 picosecond timescale, dominated by an Auger heating mechanism. Elucidation of the structural deformations associated with the surface trapping of hot holes provides atomic-scale insights into the mechanisms deteriorating optoelectronic performance and a pathway towards minimizing these losses in nanocrystal devices.

  View details for DOI 10.1038/s41467-021-22116-0

  View details for PubMedID 33767138

• Electron-ion coincidence measurements of molecular dynamics with intense X-ray pulses. Scientific reports Li, X., Inhester, L., Osipov, T., Boll, R., Coffee, R., Cryan, J., Gatton, A., Gorkhover, T., Hartman, G., Ilchen, M., Knie, A., Lin, M., Minitti, M. P., Weninger, C., Wolf, T. J., Son, S., Santra, R., Rolles, D., Rudenko, A., Walter, P. 2021; 11 (1): 505

  Abstract

  Molecules can sequentially absorb multiple photons when irradiated by an intense X-ray pulse from a free-electron laser. If the time delay between two photoabsorption events can be determined, this enables pump-probe experiments with a single X-ray pulse, where the absorption of the first photon induces electronic and nuclear dynamics that are probed by the absorption of the second photon. Here we show a realization of such a single-pulse X-ray pump-probe scheme on N[Formula: see text] molecules, using the X-ray induced dissociation process as an internal clock that is read out via coincident detection of photoelectrons and fragment ions. By coincidence analysis of the kinetic energies of the ionic fragments and photoelectrons, the transition from a bound molecular dication to two isolated atomic ions is observed through the energy shift of the inner-shell electrons. Via ab-initio simulations, we are able to map characteristic features in the kinetic energy release and photoelectron spectrum to specific delay times between photoabsorptions. In contrast to previous studies where nuclear motions were typically revealed by measuring ion kinetics, our work shows that inner-shell photoelectron energies can also be sensitive probes of nuclear dynamics, which adds one more dimension to the study of light-matter interactions with X-ray pulses.

  View details for DOI 10.1038/s41598-020-79818-6

  View details for PubMedID 33436816

• Conformer-specific photochemistry imaged in real space and time. Science (New York, N.Y.) Champenois, E. G., Sanchez, D. M., Yang, J., Figueira Nunes, J. P., Attar, A., Centurion, M., Forbes, R., Gühr, M., Hegazy, K., Ji, F., Saha, S. K., Liu, Y., Lin, M. F., Luo, D., Moore, B., Shen, X., Ware, M. R., Wang, X. J., Martínez, T. J., Wolf, T. J. 2021; 374 (6564): 178-182

  Abstract

  [Figure: see text].

  View details for DOI 10.1126/science.abk3132

  View details for PubMedID 34618569

• Carrier-specific dynamics in 2H-MoTe2 observed by femtosecond soft x-ray absorption spectroscopy using an x-ray free-electron laser. Structural dynamics (Melville, N.Y.) Britz, A., Attar, A. R., Zhang, X., Chang, H., Nyby, C., Krishnamoorthy, A., Park, S. H., Kwon, S., Kim, M., Nordlund, D., Sainio, S., Heinz, T. F., Leone, S. R., Lindenberg, A. M., Nakano, A., Ajayan, P., Vashishta, P., Fritz, D., Lin, M., Bergmann, U. 2021; 8 (1): 014501

  Abstract

  Femtosecond carrier dynamics in layered 2H-MoTe2 semiconductor crystals have been investigated using soft x-ray transient absorption spectroscopy at the x-ray free-electron laser (XFEL) of the Pohang Accelerator Laboratory. Following above-bandgap optical excitation of 2H-MoTe2, the photoexcited hole distribution is directly probed via short-lived transitions from the Te 3d 5/2 core level (M5-edge, 572-577eV) to transiently unoccupied states in the valence band. The optically excited electrons are separately probed via the reduced absorption probability at the Te M5-edge involving partially occupied states of the conduction band. A 400±110 fs delay is observed between this transient electron signal near the conduction band minimum compared to higher-lying states within the conduction band, which we assign to hot electron relaxation. Additionally, the transient absorption signals below and above the Te M5 edge, assigned to photoexcited holes and electrons, respectively, are observed to decay concomitantly on a 1-2 ps timescale, which is interpreted as electron-hole recombination. The present work provides a benchmark for applications of XFELs for soft x-ray absorption studies of carrier-specific dynamics in semiconductors, and future opportunities enabled by this method are discussed.

  View details for DOI 10.1063/4.0000048

  View details for PubMedID 33511247

• Structure retrieval in liquid-phase electron scattering. Physical chemistry chemical physics : PCCP Yang, J., Nunes, J. P., Ledbetter, K., Biasin, E., Centurion, M., Chen, Z., Cordones, A. A., Crissman, C., Deponte, D. P., Glenzer, S. H., Lin, M., Mo, M., Rankine, C. D., Shen, X., Wolf, T. J., Wang, X. 2020

  Abstract

  Electron scattering on liquid samples has been enabled recently by the development of ultrathin liquid sheet technologies. The data treatment of liquid-phase electron scattering has been mostly reliant on methodologies developed for gas electron diffraction, in which theoretical inputs and empirical fittings are often needed to account for the atomic form factor and remove the inelastic scattering background. In this work, we present an alternative data treatment method that is able to retrieve the radial distribution of all the charged particle pairs without the need of either theoretical inputs or empirical fittings. The merits of this new method are illustrated through the retrieval of real-space molecular structure from experimental electron scattering patterns of liquid water, carbon tetrachloride, chloroform, and dichloromethane.

  View details for DOI 10.1039/d0cp06045c

  View details for PubMedID 33367391

• Photodissociation of aqueous I3- observed with liquid-phase ultrafast mega-electronvolt electron diffraction Structural Dynamics Ledbetter, K., et al 2020; 21: 10

  Abstract

  Developing femtosecond resolution methods for directly observing structural dynamics is critical to understanding complex photochemical reaction mechanisms in solution. We have used two recent developments, ultrafast mega-electron-volt electron sources and vacuum compatible sub-micron thick liquid sheet jets, to enable liquid-phase ultrafast electron diffraction (LUED). We have demonstrated the viability of LUED by investigating the photodissociation of tri-iodide initiated with a 400 nm laser pulse. This has enabled the average speed of the bond expansion to be measured during the first 750 fs of dissociation and the geminate recombination to be directly captured on the picosecond time scale.

  View details for DOI 10.1063/4.0000051

  View details for PubMedCentralID PMC7771998

• Simultaneous Observation of Carrier-Specific Redistribution and Coherent Lattice Dynamics in 2H-MoTe2 with Femtosecond Core-Level Spectroscopy. ACS nano Attar, A. R., Chang, H., Britz, A., Zhang, X., Lin, M., Krishnamoorthy, A., Linker, T., Fritz, D., Neumark, D. M., Kalia, R. K., Nakano, A., Ajayan, P., Vashishta, P., Bergmann, U., Leone, S. R. 2020

  Abstract

  We employ few-femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to reveal simultaneously the intra- and interband carrier relaxation and the light-induced structural dynamics in nanoscale thin films of layered 2H-MoTe2 semiconductor. By interrogating the valence electronic structure via localized Te 4d (39-46 eV) and Mo 4p (35-38 eV) core levels, the relaxation of the photoexcited hole distribution is directly observed in real time. We obtain hole thermalization and cooling times of 15 ± 5 fs and 380 ± 90 fs, respectively, and an electron-hole recombination time of 1.5 ± 0.1 ps. Furthermore, excitations of coherent out-of-plane A1g (5.1 THz) and in-plane E1g (3.7 THz) lattice vibrations are visualized through oscillations in the XUV absorption spectra. By comparison to Bethe-Salpeter equation simulations, the spectral changes are mapped to real-space excited-state displacements of the lattice along the dominant A1g coordinate. By directly and simultaneously probing the excited carrier distribution dynamics and accompanying femtosecond lattice displacement in 2H-MoTe2 within a single experiment, our work provides a benchmark for understanding the interplay between electronic and structural dynamics in photoexcited nanomaterials.

  View details for DOI 10.1021/acsnano.0c06988

  View details for PubMedID 33085888

• Tunable isolated attosecond X-ray pulses with gigawatt peak power from a free-electron laser NATURE PHOTONICS Duris, J., Li, S., Driver, T., Champenois, E. G., MacArthur, J. P., Lutman, A. A., Zhang, Z., Rosenberger, P., Aldrich, J. W., Coffee, R., Coslovich, G., Decker, F., Glownia, J. M., Hartmann, G., Helml, W., Kamalov, A., Knurr, J., Krzywinski, J., Lin, M., Nantel, M., Natan, A., O'Neal, J., Shivaram, N., Walter, P., Wang, A., Welch, J. J., Wolf, T. A., Xu, J. Z., Kling, M. F., Bucksbaum, P. H., Zholents, A., Huang, Z., Cryan, J. P., Marinelli, A., Marangos, J. P. 2020; 14 (1): 30-+

  View details for DOI 10.1038/s41566-019-0549-5

  View details for Web of Science ID 000504727600007

• Optical Control of Non-Equilibrium Phonon Dynamics. Nano letters Krishnamoorthy, A., Lin, M., Zhang, X., Weninger, C., Ma, R., Britz, A., Tiwary, C. S., Kochat, V., Apte, A., Yang, J., Park, S., Li, R., Shen, X., Wang, X., Kalia, R., Nakano, A., Shimojo, F., Fritz, D., Bergmann, U., Ajayan, P., Vashishta, P. 2019

  Abstract

  The light-induced selective population of short-lived far-from-equilibrium vibration modes is a promising approach for controlling ultrafast and irreversible structural changes in functional nanomaterials. However, this requires a detailed understanding of the dynamics and evolution of these phonon modes and their coupling to the excited-state electronic structure. Here, we combine femtosecond mega-electronvolt electron diffraction experiments on a prototypical layered material, MoTe2, with non-adiabatic quantum molecular dynamics simulations and ab initio electronic structure calculations to show how non-radiative energy relaxation pathways for excited electrons can be tuned by controlling the optical excitation energy. We show how the dominant intravalley and intervalley scattering mechanisms for hot and band-edge electrons leads to markedly different transient phonon populations evident in electron diffraction patterns. This understanding of how tuning optical excitations affect phonon populations and atomic motion is critical for efficiently controlling light-induced structural transitions of optoelectronic devices.

  View details for DOI 10.1021/acs.nanolett.9b01179

  View details for PubMedID 31260315

• Phonon-Suppressed Auger Scattering of Charge Carriers in Defective Two-Dimensional Transition Metal Dichalcogenides. Nano letters Li, L. n., Lin, M. F., Zhang, X. n., Britz, A. n., Krishnamoorthy, A. n., Ma, R. n., Kalia, R. K., Nakano, A. n., Vashishta, P. n., Ajayan, P. n., Hoffmann, M. C., Fritz, D. M., Bergmann, U. n., Prezhdo, O. V. 2019

  Abstract

  Two-dimensional transition metal dichalcogenides (TMDs) draw strong interest in materials science, with applications in optoelectronics and many other fields. Good performance requires high carrier concentrations and long lifetimes. However, high concentrations accelerate energy exchange between charged particles by Auger-type processes, especially in TMDs where many-body interactions are strong, thus facilitating carrier trapping. We report time-resolved optical pump-THz probe measurements of carrier lifetimes as a function of carrier density. Surprisingly, the lifetime reduction with increased density is very weak. It decreases only by 20% when we increase the pump fluence 100 times. This unexpected feature of the Auger process is rationalized by our time-domain ab initio simulations. The simulations show that phonon-driven trapping competes successfully with the Auger process. On the one hand, trap states are relatively close to band edges, and phonons accommodate efficiently the electronic energy during the trapping. On the other hand, trap states localize around defects, and the overlap of trapped and free carriers is small, decreasing carrier-carrier interactions. At low carrier densities, phonons provide the main charge trapping mechanism, decreasing carrier lifetimes compared to defect-free samples. At high carrier densities, phonons suppress Auger processes and lower the dependence of the trapping rate on carrier density. Our results provide theoretical insights into the diverse roles played by phonons and Auger processes in TMDs and generate guidelines for defect engineering to improve device performance at high carrier densities.

  View details for DOI 10.1021/acs.nanolett.9b02005

  View details for PubMedID 31434484

• Tabletop Femtosecond M-edge X-ray Absorption Near-Edge Structure of FeTPPCI: Metalloporphyrin Photophysics from the Perspective of the Metal JOURNAL OF THE AMERICAN CHEMICAL SOCIETY Ryland, E. S., Lin, M., Verkamp, M. A., Zhang, K., Benke, K., Carlson, M., Vura-Weis, J. 2018; 140 (13): 4691–96

  Abstract

  Iron porphyrins are the active sites of many natural and artificial catalysts, and their photoinduced dynamics have been described as either relaxation into a vibrationally hot ground state or as a cascade through metal-centered states. In this work, we directly probe the metal center of iron(III) tetraphenyl porphyrin chloride (FeTPPCl) using femtosecond M2,3-edge X-ray absorption near-edge structure (XANES) spectroscopy. Photoexcitation at 400 nm produces a (π,π*) state that evolves in 70 fs to an iron(II) ligand-to-metal charge transfer (LMCT) state. The LMCT state relaxes to a vibrationally hot ground state in 1.13 ps, without involvement of (d,d) intermediates. The tabletop extreme-ultraviolet probe, combined with semiempirical ligand field multiplet calculations, clearly distinguishes between metal-centered and ligand-centered excited states and resolves competing accounts of Fe(III) porphyrin relaxation. This work introduces tabletop M-edge XANES as a valuable tool for measuring femtosecond dynamics of molecular transition metal complexes in the condensed phase.

  View details for DOI 10.1021/jacs.8b01101

  View details for Web of Science ID 000429508600034

  View details for PubMedID 29537834

• LCLS in-photon out: fluorescence measurement of neon using soft x-rays JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS Obaid, R., Buth, C., Dakovski, G. L., Beerwerth, R., Holmes, M., Aldrich, J., Lin, M., Minitti, M., Osipov, T., Schlotter, W., Cederbaum, L. S., Fritzsche, S., Berrah, N. 2018; 51 (3)

  View details for DOI 10.1088/1361-6455/aaa189

  View details for Web of Science ID 000419798400001

• Carrier-Specific Femtosecond XUV Transient Absorption of PbI2 Reveals Ultrafast Nonradiative Recombination JOURNAL OF PHYSICAL CHEMISTRY C Lin, M., Verkamp, M. A., Leveillee, J., Ryland, E. S., Benke, K., Zhang, K., Weninger, C., Shen, X., Li, R., Fritz, D., Bergmann, U., Wang, X., Schleife, A., Vura-Weis, J. 2017; 121 (50): 27886–93

  View details for DOI 10.1021/acs.jpcc.7b11147

  View details for Web of Science ID 000418784100012

• Ultrafast non-radiative dynamics of atomically thin MoSe2 NATURE COMMUNICATIONS Lin, M., Kochat, V., Krishnamoorthy, A., Bassman, L., Weninger, C., Zheng, Q., Zhang, X., Apte, A., Tiwary, C., Shen, X., Li, R., Kalia, R., Ajayan, P., Nakano, A., Vashishta, P., Shimojo, F., Wang, X., Fritz, D. M., Bergmann, U. 2017; 8: 1745

  Abstract

  Photo-induced non-radiative energy dissipation is a potential pathway to induce structural-phase transitions in two-dimensional materials. For advancing this field, a quantitative understanding of real-time atomic motion and lattice temperature is required. However, this understanding has been incomplete due to a lack of suitable experimental techniques. Here, we use ultrafast electron diffraction to directly probe the subpicosecond conversion of photoenergy to lattice vibrations in a model bilayered semiconductor, molybdenum diselenide. We find that when creating a high charge carrier density, the energy is efficiently transferred to the lattice within one picosecond. First-principles nonadiabatic quantum molecular dynamics simulations reproduce the observed ultrafast increase in lattice temperature and the corresponding conversion of photoenergy to lattice vibrations. Nonadiabatic quantum simulations further suggest that a softening of vibrational modes in the excited state is involved in efficient and rapid energy transfer between the electronic system and the lattice.

  View details for DOI 10.1038/s41467-017-01844-2

  View details for Web of Science ID 000416229300031

  View details for PubMedID 29170416

  View details for PubMedCentralID PMC5701075

• A novel method for resonant inelastic soft X-ray scattering via photoelectron spectroscopy detection JOURNAL OF SYNCHROTRON RADIATION Dakovski, G. L., Lin, M., Damiani, D. S., Schlotter, W. F., Turner, J. J., Nordlund, D., Ogasawara, H. 2017; 24: 1180–86

  Abstract

  A method for measuring resonant inelastic X-ray scattering based on the conversion of X-ray photons into photoelectrons is presented. The setup is compact, relies on commercially available detectors, and offers significant flexibility. This method is demonstrated at the Linac Coherent Light Source with ∼0.5 eV resolution at the cobalt L3-edge, with signal rates comparable with traditional grating spectrometers.

  View details for PubMedID 29091061

• Impact of spatial chirp on high-harmonic extreme ultraviolet absorption spectroscopy of thin films JOURNAL OF THE OPTICAL SOCIETY OF AMERICA B-OPTICAL PHYSICS Lin, M., Verkamp, M. A., Ryland, E. S., Zhang, K., Vura-Weis, J. 2016; 33 (9): 1986–92

  View details for DOI 10.1364/JOSAB.33.001986

  View details for Web of Science ID 000385419900027

• Shrinking the Synchrotron: Tabletop Extreme Ultraviolet Absorption of Transition-Metal Complexes JOURNAL OF PHYSICAL CHEMISTRY LETTERS Zhang, K., Lin, M., Ryland, E. S., Verkamp, M. A., Benke, K., de Groot, F. F., Girolami, G. S., Vura-Weis, J. 2016; 7 (17): 3383–87

  Abstract

  We show that the electronic structure of molecular first-row transition-metal complexes can be reliably measured using tabletop high-harmonic XANES at the metal M2,3 edge. Extreme ultraviolet photons in the 50-70 eV energy range probe 3p → 3d transitions, with the same selection rules as soft X-ray L2,3-edge absorption (2p → 3d excitation). Absorption spectra of model complexes are sensitive to the electronic structure of the metal center, and ligand field multiplet simulations match the shapes and peak-to-peak spacings of the experimental spectra. This work establishes high-harmonic spectroscopy as a powerful tool for studying the electronic structure of molecular inorganic, bioinorganic, and organometallic compounds.

  View details for DOI 10.1021/acs.jpclett.6b01393

  View details for Web of Science ID 000382603300015

  View details for PubMedID 27513100

• Sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy setup for pulsed and constant wave X-ray light sources REVIEW OF SCIENTIFIC INSTRUMENTS Shavorskiy, A., Neppl, S., Slaughter, D. S., Cryan, J. P., Siefermann, K. R., Weise, F., Lin, M., Bacellar, C., Ziemkiewicz, M. P., Zegkinoglou, I., Fraund, M. W., Khurmi, C., Hertlein, M. P., Wright, T. W., Huse, N., Schoenlein, R. W., Tyliszczak, T., Coslovich, G., Robinson, J., Kaindl, R. A., Rude, B. S., Oelsner, A., Maehl, S., Bluhm, H., Gessner, O. 2014; 85 (9): 093102

  Abstract

  An apparatus for sub-nanosecond time-resolved ambient-pressure X-ray photoelectron spectroscopy studies with pulsed and constant wave X-ray light sources is presented. A differentially pumped hemispherical electron analyzer is equipped with a delay-line detector that simultaneously records the position and arrival time of every single electron at the exit aperture of the hemisphere with ~0.1 mm spatial resolution and ~150 ps temporal accuracy. The kinetic energies of the photoelectrons are encoded in the hit positions along the dispersive axis of the two-dimensional detector. Pump-probe time-delays are provided by the electron arrival times relative to the pump pulse timing. An average time-resolution of (780 ± 20) ps (FWHM) is demonstrated for a hemisphere pass energy E(p) = 150 eV and an electron kinetic energy range KE = 503-508 eV. The time-resolution of the setup is limited by the electron time-of-flight (TOF) spread related to the electron trajectory distribution within the analyzer hemisphere and within the electrostatic lens system that images the interaction volume onto the hemisphere entrance slit. The TOF spread for electrons with KE = 430 eV varies between ~9 ns at a pass energy of 50 eV and ~1 ns at pass energies between 200 eV and 400 eV. The correlation between the retarding ratio and the TOF spread is evaluated by means of both analytical descriptions of the electron trajectories within the analyzer hemisphere and computer simulations of the entire trajectories including the electrostatic lens system. In agreement with previous studies, we find that the by far dominant contribution to the TOF spread is acquired within the hemisphere. However, both experiment and computer simulations show that the lens system indirectly affects the time resolution of the setup to a significant extent by inducing a strong dependence of the angular spread of electron trajectories entering the hemisphere on the retarding ratio. The scaling of the angular spread with the retarding ratio can be well approximated by applying Liouville's theorem of constant emittance to the electron trajectories inside the lens system. The performance of the setup is demonstrated by characterizing the laser fluence-dependent transient surface photovoltage response of a laser-excited Si(100) sample.

  View details for PubMedID 25273702

• Atomic-Scale Perspective of Ultrafast Charge Transfer at a Dye-Semiconductor Interface JOURNAL OF PHYSICAL CHEMISTRY LETTERS Siefermann, K. R., Pemmaraju, C. D., Neppl, S., Shavorskiy, A., Cordones, A. A., Vura-Weis, J., Slaughter, D. S., Sturm, F. P., Weise, F., Bluhm, H., Strader, M. L., Cho, H., Lin, M., Bacellar, C., Khurmi, C., Guo, J., Coslovich, G., Robinson, J. S., Kaindl, R. A., Schoenlein, R. W., Belkacem, A., Neumark, D. M., Leone, S. R., Nordlund, D., Ogasawara, H., Krupin, O., Turner, J. J., Schlotter, W. F., Holmes, M. R., Messerschmidt, M., Minitti, M. P., Gul, S., Zhang, J. Z., Huse, N., Prendergast, D., Gessner, O. 2014; 5 (15): 2753-2759

  Abstract

  Understanding interfacial charge-transfer processes on the atomic level is crucial to support the rational design of energy-challenge relevant systems such as solar cells, batteries, and photocatalysts. A femtosecond time-resolved core-level photoelectron spectroscopy study is performed that probes the electronic structure of the interface between ruthenium-based N3 dye molecules and ZnO nanocrystals within the first picosecond after photoexcitation and from the unique perspective of the Ru reporter atom at the center of the dye. A transient chemical shift of the Ru 3d inner-shell photolines by (2.3 ± 0.2) eV to higher binding energies is observed 500 fs after photoexcitation of the dye. The experimental results are interpreted with the aid of ab initio calculations using constrained density functional theory. Strong indications for the formation of an interfacial charge-transfer state are presented, providing direct insight into a transient electronic configuration that may limit the efficiency of photoinduced free charge-carrier generation.

  View details for DOI 10.1021/jz501264x

  View details for Web of Science ID 000340222200044

  View details for PubMedID 26277975

• Ionization and dissociation dynamics of vinyl bromide probed by femtosecond extreme ultraviolet transient absorption spectroscopy JOURNAL OF CHEMICAL PHYSICS Lin, M., Neumark, D. M., Gessner, O., Leone, S. R. 2014; 140 (6): 064311

  Abstract

  Strong-field induced ionization and dissociation dynamics of vinyl bromide, CH2=CHBr, are probed using femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy. Strong-field ionization is initiated with an intense femtosecond, near infrared (NIR, 775 nm) laser field. Femtosecond XUV pulses covering the photon energy range of 50-72 eV probe the subsequent dynamics by measuring the time-dependent spectroscopic features associated with transitions of the Br (3d) inner-shell electrons to vacancies in molecular and atomic valence orbitals. Spectral signatures are observed for the depletion of neutral C2H3Br, the formation of C2H3Br(+) ions in their ground (X̃) and first excited (Ã) states, the production of C2H3Br(++) ions, and the appearance of neutral Br ((2)P3/2) atoms by dissociative ionization. The formation of free Br ((2)P3/2) atoms occurs on a timescale of 330 ± 150 fs. The ionic à state exhibits a time-dependent XUV absorption energy shift of ∼0.4 eV within the time window of the atomic Br formation. The yield of Br atoms correlates with the yield of parent ions in the à state as a function of NIR peak intensity. The observations suggest that a fraction of vibrationally excited C2H3Br(+) (Ã) ions undergoes intramolecular vibrational energy redistribution followed by the C-Br bond dissociation. The C2H3Br(+) (X̃) products and the majority of the C2H3Br(++) ions are relatively stable due to a deeper potential well and a high dissociation barrier, respectively. The results offer powerful new insights about orbital-specific electronic processes in high field ionization, coupled vibrational relaxation and dissociation dynamics, and the correlation of valence hole-state location and dissociation in polyatomic molecules, all probed simultaneously by ultrafast table-top XUV spectroscopy.

  View details for DOI 10.1063/1.4865128

  View details for Web of Science ID 000331868100026

  View details for PubMedID 24527919

• Strong-field induced XUV transmission and multiplet splitting in 4d(-1)6p core-excited Xe studied by femtosecond XUV transient absorption spectroscopy JOURNAL OF CHEMICAL PHYSICS Lin, M., Pfeiffer, A. N., Neumark, D. M., Leone, S. R., Gessner, O. 2012; 137 (24): 244305

  Abstract

  Light-induced coupling of core-excited states of Xe atoms is investigated by femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy with photon energies ranging from 50 eV to 72 eV. Coupling of the 4d(-1)((2)D(5/2))6p((2)P(3/2)) (65.1 eV) and 4d(-1)((2)D(3/2))6p((2)P(1/2)) (67.0 eV) core-excited states to nearby states by a strong infrared laser field leads to a threefold enhancement of XUV transmission. The transmission at 65.1 eV (67.0 eV) changes from 3.2 ± 0.4% (5.9 ± 0.5%) without the coupling laser to 9 ± 2% (22 ± 5%) at the maximum of the laser field. A strong-field induced broad XUV absorption feature between 60 eV and 65 eV is ascribed to splitting of the field-free absorption lines into multiple branches when the Rabi frequencies of the coupling transitions exceed the infrared laser frequency. This picture is supported by a comparison of the strong-field induced absorption spectrum with a numerical integration of the von Neumann equation for a few-level quantum system. The valence hole-alignment of strong-field ionized Xe is revisited, confirming the previously observed reduced alignment compared to theoretical predictions.

  View details for DOI 10.1063/1.4772199

  View details for Web of Science ID 000312832100014

  View details for PubMedID 23277934

• Photostability of amino acids: photodissociation dynamics of phenylalanine chromophores PHYSICAL CHEMISTRY CHEMICAL PHYSICS Tseng, C., Lin, M., Yang, Y., Ho, Y., Ni, C., Chang, J. 2010; 12 (19): 4989–95

  Abstract

  The theoretical prediction of H atom elimination on the excited state of phenol, imidazole and indole, the respective chromophores for the amino acids tyrosine, histidine and tryptophan, and the confirmation of theoretical prediction by experimental observations have a great impact on the explanation of photostability of amino acids upon irradiation with UV photons. On the other hand, no theoretical prediction of the excited state photodissociation dynamics has been made on the other aromatic amino acid, phenylalanine. In this work, photodissociation dynamics for various phenylalanine chromophores, including, phenylethylamine, N-methyl-phenylethylamine, and N-acetyl phenylalanine methyl ester was investigated in a molecular beam at 248 and 193 nm using multimass ion imaging techniques. The major dissociation channel for these compounds is the C-C bond cleavage. However, the photofragment translational energy distribution of phenylethylamine contains two components. The slow component corresponds to the dissociation on the ground state surface after internal conversion, and the fast component represents the dissociation from an excited state with a large exit barrier. The competition between the dissociation on the ground state and on the excited state changes as the size of chromophores increases. Internal conversion to the ground state prior to dissociation becomes the major nonradiative process for large chromophores. This study reveals the size-dependent photostability for these amino acid chromophores.

  View details for DOI 10.1039/b925338f

  View details for Web of Science ID 000277359300011

  View details for PubMedID 20405063

• Photodissociation dynamics of small aromatic molecules studied by multimass ion imaging JOURNAL OF PHYSICAL CHEMISTRY B Ni, C., Tseng, C., Lin, M., Dyakov, Y. A. 2007; 111 (44): 12631–42

  Abstract

  The photodissociation dynamics of various aromatic molecules, studied using multimass ion imaging techniques, is reviewed. The experimental data reveals new isomerization and dissociation mechanisms. Our investigation of benzene, pyridine, and pyrimidine finds that H-atom elimination thresholds remain the same for the three molecules. We also notice that ring-opening dissociation thresholds decrease rapidly with the increase of the number of nitrogen atoms in the aromatic ring. Hydrogen atom elimination is the sole dissociation channel for benzene at 193 nm. Along with H-atom elimination, we observe five distinct ring-opening dissociation channels for pyridine at 193 nm. No dissociation channels were observed for benzene and pyridine at 248 nm. Ring-opening dissociation channels are the major channels for pyrimidine, which dissociates at 193 nm and also at 248 nm. A six-membered to seven-membered ring isomerization was observed for photodissociation processes involving toluene, m-xylene, aniline, 4-methylpyridine, alpha-fluorotoluene, and 4-fluorotoluene, indicating a general isomerization mechanism for all such aromatic molecules. What is significant, is that during the isomerization, atoms (i.e., carbon, nitrogen, fluorine, and hydrogen) belonging to respective alkyl or amino groups are involved in an exchange with atoms within the aromatic ring. This type of isomerization is not observed in other aromatic isomerization mechanisms. For small tyrosine chromophores, such as phenol, 4-methylphenol, and 4-ethylphenol, H-atom elimination from a repulsive excited state plays a key role. However, dissociation is quenched in large chromophores like 4-(2-aminoethyl)-phenol. Our work demonstrates the capability and high sensitivity of multimass ion imaging techniques in the study of aromatic compounds.

  View details for DOI 10.1021/jp074904j

  View details for Web of Science ID 000250650800003

  View details for PubMedID 17935318

• Photodissociation dynamics of phenol JOURNAL OF PHYSICAL CHEMISTRY A Tseng, C., Lee, Y. T., Lin, M., Ni, C., Liu, S., Lee, Y., Xu, Z. F., Lin, M. C. 2007; 111 (38): 9463–70

  Abstract

  The photodissociation of phenol at 193 and 248 nm was studied using multimass ion-imaging techniques and step-scan time-resolved Fourier-transform spectroscopy. The major dissociation channels at 193 nm include cleavage of the OH bond, elimination of CO, and elimination of H(2)O. Only the former two channels are observed at 248 nm. The translational energy distribution shows that H-atom elimination occurs in both the electronically excited and ground states, but elimination of CO or H(2)O occurs in the electronic ground state. Rotationally resolved emission spectra of CO (1

  View details for DOI 10.1021/jp073282z

  View details for Web of Science ID 000249655600038

  View details for PubMedID 17691716

• Photodissociation of S atom containing amino acid chromophores JOURNAL OF CHEMICAL PHYSICS Lin, M., Dyakov, Y. A., Lee, Y. T., Lin, S. H., Mebel, A. M., Ni, C. 2007; 127 (6): 064308

  Abstract

  Photodissociation of 3-(methylthio)propylamine and cysteamine, the chromophores of S atom containing amino acid methionine and cysteine, respectively, was studied separately in a molecular beam at 193 nm using multimass ion imaging techniques. Four dissociation channels were observed for 3-(methylthio)propylamine, including (1) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)SCH(2)CH(2)CH(2)NH+H, (2) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)+SCH(2)CH(2)CH(2)NH(2), (3) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)S+CH(2)CH(2)CH(2)NH(2), and (4) CH(3)SCH(2)CH(2)CH(2)NH(2)-->CH(3)SCH(2)+CH(2)CH(2)NH(2). Two dissociation channels were observed from cysteamine, including (5) HSCH(2)CH(2)NH(2)-->HS+CH(2)CH(2)NH(2) and (6) HSCH(2)CH(2)NH(2)-->HSCH(2)+CH(2)NH(2). The photofragment translational energy distributions suggest that reaction (1) and parts of the reactions (2), (3), (5) occur on the repulsive excited states. However, reaction (4), (6) occur only after the internal conversion to the electronic ground state. Since the dissociation from an excited state with a repulsive potential energy surface is very fast, it would not be quenched completely even in the condensed phase. Our results indicate that reactions following dissociation may play an important role in the UV photochemistry of S atom containing amino acid chromophores in the condensed phase. A comparison with the potential energy surface from ab initio calculations and branching ratios from RRKM calculations was made.

  View details for DOI 10.1063/1.2761916

  View details for Web of Science ID 000248760300016

  View details for PubMedID 17705597

• Photostability of amino acids: Internal conversion versus dissociation JOURNAL OF CHEMICAL PHYSICS Lin, M., Tzeng, C., Dyakov, Y. A., Ni, C. 2007; 126 (24): 241104

  Abstract

  Photodissociation dynamics for various tryptophan chromophores was studied at 193 or 248 nm using multimass ion imaging techniques. The competition between internal conversion to the ground electronic state and dissociation from the repulsive excited state reveals size-dependent photostability for these amino acid chromophores. As the size of chromophore increases, internal conversion to the ground state becomes the major nonradiative process. For tryptophan and larger chromophores, dissociation directly from the repulsive state is completely quenched.

  View details for DOI 10.1063/1.2751150

  View details for Web of Science ID 000247625800006

  View details for PubMedID 17614530

• Photodissociation dynamics of nitrobenzene and o-nitrotoluene JOURNAL OF CHEMICAL PHYSICS Lin, M., Lee, Y. T., Ni, C., Xu, S., Lin, M. C. 2007; 126 (6): 064310

  Abstract

  Photodissociation of nitrobenzene at 193, 248, and 266 nm and o-nitrotoluene at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Fragments corresponding to NO and NO(2) elimination from both nitrobenzene and o-nitrotoluene were observed. The translational energy distributions for the NO elimination channel show bimodal distributions, indicating two dissociation mechanisms involved in the dissociation process. The branching ratios between NO and NO(2) elimination channels were determined to be NONO(2)=0.32+/-0.12 (193 nm), 0.26+/-0.12 (248 nm), and 0.4+/-0.12(266 nm) for nitrobenzene and 0.42+/-0.12(193 nm) and 0.3+/-0.12 (248 nm) for o-nitrotoluene. Additional dissociation channels, O atom elimination from nitrobenzene, and OH elimination from o-nitrotoluene, were observed. New dissociation mechanisms were proposed, and the results are compared with potential energy surfaces obtained from ab initio calculations. Observed absorption bands of photodissociation are assigned by the assistance of the ab initio calculations for the relative energies of the triplet excited states and the vertical excitation energies of the singlet and triplet excited states of nitrobenzene and o-nitrotoluene. Finally, the dissociation rates and lifetimes of photodissociation of nitrobenzene and o-nitrotoluene were predicted and compared to experimental results.

  View details for DOI 10.1063/1.2435351

  View details for Web of Science ID 000244250200014

  View details for PubMedID 17313218

• Photodissociation dynamics of pyrimidine JOURNAL OF CHEMICAL PHYSICS Lin, M. F., Dyakov, Y. A., Tseng, C. M., Mebel, A. M., Lin, S. H., Lee, Y. T., Ni, C. K. 2006; 124 (8): 084303

  Abstract

  Photodissociation of pyrimidine at 193 and 248 nm was investigated separately using vacuum ultraviolet photoionization at 118.4 and 88.6 nm and multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C4N2H4 --> C4N2H3 + H and five ring opening dissociation channels, C4N2H4 --> C3NH3 + HCN, C4N2H4 --> 2C2NH2, C4N2H4 --> CH3N + C3NH, C4N2H4 --> C4NH2 + NH2, and C4N2H4 --> CH2N + C3NH2. Only the first four channels were observed at 248 nm. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion at both wavelengths. The dissociation rates were found to be >5 x 10(7) and 1 x 10(6) s(-1) at 193 and 248 nm, respectively. Comparison with the potential energies from ab initio calculations have been made.

  View details for DOI 10.1063/1.2174011

  View details for Web of Science ID 000235663300018

  View details for PubMedID 16512712

• Photodissociation and photoisomerization of α-fluorotoluene and 4-fluorotoluene in a molecular beam The Journal of Chemical Physics Huang, C., Jiang, J., Dyakov, Y., Lin, M., Tseng, C., Lin, S., Lee, Y., Ni, C. 2006; 125: 133305
• Photodissociation dynamics of indole in a molecular beam JOURNAL OF CHEMICAL PHYSICS Lin, M. F., Tseng, C. M., Lee, Y. T., Ni, C. K. 2005; 123 (12): 124303

  Abstract

  Photodissociation of indole at 193 and 248 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. H atom elimination was found to be the only dissociation channel at both wavelengths. The photofragment translational energy distribution obtained at 193 nm contains a fast and a slow component. Fifty-four percent of indole following the 193 nm photoexcitation dissociate from electronically excited state, resulting in the fast component. The rest of 46% indole dissociate through the ground electronic state, giving rise to the slow component. A dissociation rate of 6 x 10(5) s(-1), corresponding to the dissociation from the ground electronic state, was determined. Similar two-component translational energy distribution was observed at 248 nm. However, more than 80% of indole dissociate from electronically excited state after the absorption of 248 nm photons. A comparison with the potential energy surfaces from the ab initio calculation has been made.

  View details for DOI 10.1063/1.2009736

  View details for Web of Science ID 000232206500014

  View details for PubMedID 16392478

• Photodissociation dynamics of pyridine JOURNAL OF CHEMICAL PHYSICS Lin, M. F., Dyakov, Y. A., Tseng, C. M., Mebel, A. M., Lin, S. H., Lee, Y. T., Ni, C. K. 2005; 123 (5): 054309

  Abstract

  Photodissociation of pyridine, 2,6-d2-pyridine, and d5-pyridine at 193 and 248 nm was investigated separately using multimass ion imaging techniques. Six dissociation channels were observed at 193 nm, including C5NH5 --> C5NH4 + H (10%) and five ring opening dissociation channels, C5NH5 --> C4H4 + HCN, C5NH5 --> C3H3 + C2NH2, C5NH5 --> C2H4 +C3NH, C5NH5 --> C4NH2 + CH3 (14%), and C5NH5 --> C2H2 + C3NH3. Extensive H and D atom exchanges of 2,6-d2-pyridine prior to dissociation were observed. Photofragment translational energy distributions and dissociation rates indicate that dissociation occurs in the ground electronic state after internal conversion. The dissociation rate of pyridine excited by 248-nm photons was too slow to be measured, and the upper limit of the dissociation rate was estimated to be 2x10(3) s(-1). Comparisons with potential energies obtained from ab initio calculations and dissociation rates obtained from the Rice-Ramsperger-Kassel-Marcus theory have been made.

  View details for DOI 10.1063/1.1994849

  View details for Web of Science ID 000231168700025

  View details for PubMedID 16108641

• Photodissociation dynamics of C6HxF6-x (x=1-4) at 193 nm JOURNAL OF PHYSICAL CHEMISTRY B Lin, M. F., Dyakov, Y. A., Lin, S. H., Lee, Y. T., Ni, C. K. 2005; 109 (17): 8344–49

  Abstract

  Photodissociation of fluorine-substituted benzenes, including 1,3-difluorobenzene, 1,2,4-trifluorobenzene, 1,2,4,5-tetrafluorobenzene, and pentafluorobenzene, at 193 nm under collision-free conditions has been studied in separate experiments using multimass ion imaging techniques. HF elimination was found to be the major dissociation channel for all of these molecules. Small amounts of photofragments of C(6)H(3)F(2) and C(6)H(2)F(3) from 1,3-difluorobenzene and 1,2,4-trifluorobenzene, respectively, were also observed. They correspond to the minor dissociation channel of hydrogen elimination. Dissociation rates and fragment translational energy distributions obtained from experimental measurements suggest that HF and hydrogen elimination reactions occur in the ground electronic state. The potential energy surface obtained from ab initio calculations indicates that the four-center reaction in the ground electronic state is the major dissociation mechanism for the HF eliminations. A comparison with the RRKM calculation has been made.

  View details for DOI 10.1021/jp07647g

  View details for Web of Science ID 000228847500009

  View details for PubMedID 16851978

• Carbon-carbon bond cleavage in the photoionization of ethanol and 1-propanol clusters JOURNAL OF CHEMICAL PHYSICS Tsai, S. T., Jiang, J. C., Lin, M. F., Lee, Y. T., Ni, C. K. 2004; 120 (19): 8979–84

  Abstract

  Tunable VUV laser was used to initiate the ion-molecule reactions in the clusters of ethanol and 1-propanol by photoionization in the region between 10.49 to 10.08 eV. Ionic products were detected by the time-of-flight mass spectrometer. In addition to the protonated clusters from proton transfer reactions, the products corresponding to beta carbon-carbon bond cleavage were found to be one of the major products for small sizes of clusters. A comparison with photoionization of methanol clusters and the results of ab initio calculation has been made.

  View details for DOI 10.1063/1.1704637

  View details for Web of Science ID 000221146400011

  View details for PubMedID 15267833

• H and CH3 eliminations in the photodissociation of chlorotoluene JOURNAL OF CHEMICAL PHYSICS Lin, M. F., Huang, C. L., Kislov, V. V., Mebel, A. M., Lee, Y. T., Ni, C. K. 2003; 119 (15): 7701–4

  View details for DOI 10.1063/1.1609393

  View details for Web of Science ID 000185703100010

• Photodissociation dynamics of azulene JOURNAL OF CHEMICAL PHYSICS Lin, M. F., Huang, C. L., Lee, Y. T., Ni, C. K. 2003; 119 (4): 2032–36

  View details for DOI 10.1063/1.1584664

  View details for Web of Science ID 000184103000019